Kloek A P, Yang J, Mathews F S, Goldberg D E
Department of Medicine, Washington University School of Medicine, St. Louis, Missouri 63110.
J Biol Chem. 1993 Aug 25;268(24):17669-71.
The Ascaris perienteric hemoglobin is 10(4) times more oxygen-avid than mammalian hemoglobins. Inspection of its primary structure fails to explain this extraordinary association with oxygen. The Ascaris hemoglobin gene encodes a 40-kDa, two-domain globin; the two domains (D1 and D2) are 63% identical, and each is capable of binding a single heme. The native protein is an octamer. At the end of D2 is a highly charged carboxyl-terminal extension containing four direct repeats of HKEE. We have expressed the two domains separately in E. coli. Both individual domains are extremely oxygen-avid. D2, with attached COOH-terminal tail, is capable of multimerization, whereas D1 remains a monomer. Recombinant D1 readily forms diffractable, red, prismatic crystals. We conclude that: 1) the basis of the hemoglobin's oxygen avidity rests in an isolated heme pocket and does not involve inter-domain interactions and 2) multimerization is mediated through sequences in the second domain, most probably via the charged COOH-terminal tail.
蛔虫肠周血红蛋白对氧的亲和力比哺乳动物血红蛋白高10⁴倍。对其一级结构的研究无法解释这种与氧的非凡结合。蛔虫血红蛋白基因编码一种40 kDa的双结构域球蛋白；这两个结构域（D1和D2）有63%的同源性，且每个结构域都能结合一个血红素。天然蛋白质是八聚体。在D2末端是一个带高电荷的羧基末端延伸区，含有HKEE的四个直接重复序列。我们已在大肠杆菌中分别表达了这两个结构域。两个单独的结构域对氧的亲和力都极高。带有羧基末端尾巴的D2能够多聚化，而D1仍然是单体。重组D1很容易形成可衍射的红色棱柱形晶体。我们得出以下结论：1）血红蛋白对氧亲和力的基础在于一个孤立的血红素口袋，不涉及结构域间的相互作用；2）多聚化是通过第二个结构域中的序列介导的，很可能是通过带电荷的羧基末端尾巴。
